Tractor feed mechanisms commonly used in a printer apparatus comprise a continuous flexible pin belt wrapped around a pair of drive sprockets or pulleys rotatably supported by a frame assembly or body mountable on drive and guide bars of the paper feed assembly of the printer. In addition to supporting the pin belt and drive sprockets, the tractor body has surfaces for guiding paper so that perforations near the edges of the paper are in proper alignment to receive and be engaged by drive pins carried by the belt as it is rotated by the sprockets to feed the paper past a print station of the printer. The paper may be a continuous web and may have multiple layers in which case it is commonly called multipart forms, and the tractor feed mechanism is referred to as a forms feed tractor.
It is common practice to provide such feed mechanisms with a door or cover for constraining the paper so that it remains in contact with the guide surface of the tractor body and does not become disengaged from the pins. The door is provided with a hinge connection to the body. This enables the door to be rotated around the hinge axis between a closed position and an open position to allow paper to be assembled onto or removed from the tractor. When in the closed position the underside of the door is more or less parallel with the paper guide surface and belt, and forms one side of a slot within which the paper feed holes are engaged by the feed pins of the belt. The open position of the door can be as much as 180 degrees from the closed position, but for operator convenience in loading forms onto the tractor, the door position is commonly 90 degrees or slightly more from the closed position. Stops are commonly provided on the frame assembly or the door to limit door rotation at both the closed and open positions. Traditionally a helical spring is used to force the door against the stops with maximum extension of the spring occurring between the open and closed positions. Examples of such tractors may be seen in U.S. Pat. Nos. 3,938,721; 4,129,239 and 4,226,353 which are considered representative.
A common problem with tractors having stops for holding the door at the open position occurs when it is desired to have more than one stable open position. There is the further problem of damage occurring when an unintended force is applied in a manner which tends to force the door beyond the open stop position. An instance of a force causing damage occurs when the tractor doors are inadvertently left open and the printer is of the type having a swing gate construction. Printers having swing gate construction include the IBM 4245 and 4248 Printers. In that type printer, the tractors are mounted on a stationary frame while the type carrier, such as an engraved type band, and the ink ribbon assemblies are mounted on a swing gate which is opened by an operator to install the print forms into the tractors. Closing the gate while the tractor door remains open at or near the 90 degree position can cause damage when the swing gate is closed, which because of its superior mass and force, drives the tractor beyond the open position fixed by the stop. The damage can be especially severe when the tractor door and/or frame are made of elastomeric materials.
One solution for this problem makes use of spring biased hinges which comprise a spring acting with a detent mechanism which is part of the tractor door hinge structure. The spring may be a flat cantilever spring which is spring loaded against the detent or it may be a wire element loaded against a detent, which wire also functions as the pivot pins. In a third version, the door has pawl shaped arms and the spring means takes the form of a mass of molded elastomer having a cavity in which detent positions are formed to be engaged and deformed by rotation of the arms with the door. Examples of such structures are shown in U.S. Pat. Nos. 3,477,626; 4,614,287 and 4,650,358 and in printed publication entitled IBM 3203 Printer, Manual of Operations (S135-1003-3), pp. 70 and 71.
While such spring loaded hinge structures reduce somewhat the risk of damage to doors in the open position, they are excessively complex and costly to make and assemble. In addition, they are unable to be made relatively compact and small and still provide the level of force necessary for the door to prevent paper from being lifted from the pins as in cases where the tension and buckling of the paper tends to occur in high speed line printers using multilayer forms.
Spring loaded hinge structures are also known for use with doors on other devices such as cabinets. Examples of such structures are shown in U.S. Pat. Nos. 540,707; 2,059,582; 2,943,582; 3,842,463; 3,950,818; 4,134,516; and 4,650,358. A spring hinge structure for a tape cassette is shown in U.S. Pat. No. 4,527,755. None of these patents are concerned with the problems related to feed mechanisms such as damage to the tractor door by swing gate of a printer nor are they concerned with a problem like constraining paper forms on a pin feed belt in opposition to lifting forces generated as a result of the feeding of paper forms.